This invention relates generally to hearing aids and more particularly concerns a method and apparatus for improving hearing communication in persons with various hearing deficits. The invention will be disclosed in connection with a generalized signal processing unit which restores meaningful neural patterns to the auditory recognition centers of the brain in a manner which enables persons and other higher animals to recognize a variety of sounds including human speech.
The present invention is derived from an understanding of the physiological processes of the normal hearing apparatus, and the manner in which the auditory system is compromised in conditions of hearing loss. The hearing apparatus in humans and higher animals consists of mechanical parts which transmit acoustic pressure waves from the air to the sensory neural network. The function and pathologies of the conductive portion of the hearing apparatus are well known up to the point of the mechanical-to-neural transduction site.
The mechanical-to-neural transformation occurs within the cochlea along a structure called the basilar membrane. This membrane demonstrates excursions which have a position-to-tone relationship. A vast network of neural cells which are sensitive to displacement are disposed in rows along the length of the membrane. The membrane thus performs a spectral analysis on the incoming sound, directing various portions of the frequency spectrum along specific neural channels. The specific neural cells which are involved in this transduction process are called the inner and outer hair cells. The inner hair cells are in primary relationship with the nerves which ascend toward the brain while the outer hair cells are in primary relationship with nerves which carry information down from the brain. The exact functional role of these two neural hair cells types has not been conclusively proven. Applicant postulates that the role of the inner hair cell is that of a pure receptor, and that this cell initiates electrical signals along its neural channel to the brain when it is displaced as a result of the motion of the basilar membrane. It is further postulated that the outer hair cell exerts a control function on the inner hair cell, specifically that it alters the threshold of displacement which must be exceeded before an electrical response can be initiated along a given neural pathway. Hence the inner and outer hair cells comprise the sending and modifying functions of an adaptive control system. It follows that there is an adaptive portion to the hearing system, that it is responsible for the high degree of pitch discrimination in humans and higher animals, that disease processes affect both the receptor inner hair cells and the adaptive controller outer hair cells, and, finally, that the structure places physiological constraints on the remedial signal processing which can be performed by any hearing aid.
Conventional hearing aids serve as amplifiers of the auditory signal. The presentation of an amplified signal does provide a means for restoring the conductive functions of the ear. However, amplification alone is of only marginal help if the mechanical-to-neural structure has been damaged. In these cases the neural channels that carry specific frequencies to the brain and their contribution to the adaptivity function have been lost. Under extreme amplification, the excursion of the basilar membrane will be broad enough to excite adjacent hair cells, and if these cells exist, a signal will be sent to the brain.
However, the amplification does nothing to restore adaptivity, and the function of identifying important sounds from background noise can occur only with conscious effort at higher levels of the brain. Applicant postulates that this effort is fatiguing and is the reason why the adjustment period for a hearing aid user is difficult. In more advanced disease of the sensory-neural network, adjacent channels may not be present for stimulation by signals which are amplified to levels which can be physically damaging to the auditory structures. In these cases the portion of the frequency spectrum which has no neural channels must be readdressed to other still viable neural channels, and the adaptivity function belonging to the lost frequency portion must be performed external to the ear.
A variety of hearing aid devices have been proposed for addressing the problem of sensory-neural deafness. The methods and means of the previous methods have utilized band shifting of frequencies, various transformations of the auditory signal, alteration of the formant loci, and amplitude compression schemes. The methods fail to recognize the inseparability of the adaptive and receptor functions, and only address a part of the necessary signal processing functions which must be performed. Furthermore, the previous methods fail to recognize the highly individual nature of the hearing loss in any patient, and the need for the minimal but generalized signal processing to restore recognizable auditory patterns in the auditory cortex of the brain.
Accordingly, it is an objective of the present invention to provide new and useful methods and means for restoring recognizable auditory communication to persons having various deficits in their auditory sensory apparatus. It is another object of the present invention to define methods and means for the characterization of residual hearing function in terms of the spectral and adaptive portions of a signal processing system.
It is another objective of the present invention to provide a hearing aid system which performs signal processing on an auditory signal in a manner which compensates for the natural signal processing which is lost by the failure or destruction of parts of the normal auditory network.
It is another objective of the present invention to provide the means for automatically discriminating important information from noise in the auditory signal in an effort to restore the adaptive portion of the hearing function.
It is another objective of the present invention to provide the means for shifting, in a physiologically compatible manner, portions of the frequency spectrum of the auditory signal toward viable neural channels in the auditory organ system.